Vanadyl pyrophosphate (VO).sub.2 P.sub.2 O.sub.7 (hereinafter generally referred to as "VPO") is the active catalyst used in several commercial processes which convert butane directly to maleic anhydride via air oxidation. The catalyst is believed to perform the selective oxidation of butane via utilization of its lattice oxygen from only the outermost surface layers of the VPO crystallites. Given the accumulated evidence of only surface layer involvement in this oxidation process, there have been numerous efforts to maximize surface area of the catalyst crystallites.
The desire for high surface area has been the basis of a previous synthetic approach to obtaining VPO catalysts (see U.S. Pat. No. 4,360,453). The procedure involves a lengthy alcohol-based preparation starting from vanadium (V) oxide and uses an easily oxidized alcohol such as benzyl alcohol as a reducing agent. The procedure yields a plate-like precursor material (VO)(HPO.sub.4).cndot.0.5H.sub.2 O which may be "calcined and activated" for a long period of time in butane/air mixtures during which time a "topotactic transformation" to VPO occurs. The plate-like, crystalline morphology of the precursor is maintained during this transformation and this morphology is believed to be the reason for the high activity of catalyst prepared in this manner as compared to crystallographically identical material prepared by other (e.g., aqueous) routes.
Another related vanadium and phosphorus containing material is VO(PO.sub.3).sub.2, vanadylbis (metaphosphate). This material can also be used as a catalyst in hydrocarbon, more specifically butane, oxidation processes.
A prime characteristic of a catalytic substance, in addition to its function as a catalyst, is that it possess sufficient mechanical strength to withstand any of the forces it may be subject to during its manufacture and transport and also during its use. The catalyst must be available in a mechanical form sufficiently robust to withstand any forces due to thermal stress and/or agitation it may experience during use. Currently, known VPO catalyst precursor crystallites are spray dried with a silica precursor to form silica/VPO shell composite structures which possess improved attrition resistance when compared to non-silica strengthened VPO (see U.S. Pat. No. 4,677,084). The net result of very specific demands on catalyst physical and chemical performance characteristics is that catalyst costs are significantly increased.
Chen and Zubieta (Angew. Chem. Int. Ed. Engl. 1993, 32, 261-263) describe a phenylphosphonato cluster which has a "reentrant" vanadyl group. The exposed vanadyl site in this compound has a bound methoxy group, but potential Lewis acidity or exchange chemistry is also not discussed. Thermal decomposition is not discussed.
Chen and Zubieta (Inorg. Chem. 1993, 32, 4485-4486) describe dimeric compounds having two vanadyl groups and two bridging phosphonato groups. Here also, Lewis acidity and thermal decomposition are not discussed.
There are several other molecular organophosphorus-vanadium compounds in the open literature. However, none have a V:P ratio of 1:2. Therefore, in order to overcome some of the problems and deficiencies of the prior art, the present invention provides vanadium (IV) cluster compositions which are readily synthesized from simple reagents and which are single-molecule, highly processible precursors to vanadium-phosphorus catalyst materials. Deposition of the soluble precursors onto high surface area supports and spray drying of the soluble precursors with additional materials to produce novel catalyst composites are possible using these materials. Many of the results obtainable using the materials of the present invention are unavailable by other traditional routes to vanadium-phosphorus catalysts.
New methods for preparation of vanadium-phosphorus catalysts can have potential impact in their own right given the sensitivity of the activity and selectivity of vanadium-phosphorus catalysts to preparation protocols. Additionally, the versatility of these cluster species to possible processing alternatives such as spray drying and/or deposition onto high surface area or attrition resistant supports provides a previously unavailable route to new catalyst compositions.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.